Due to limited resources available for communication systems, active studies have been conducted on efficient use of the resources. To efficiently use the limited resources, unnecessary resource allocation should be prevented and already-allocated resources should be retrieved early and reallocated for other services.
Meanwhile, a Base Station (BS) should allocate a UL bandwidth rapidly to a Mobile Station (MS) in response to a UL bandwidth allocation request from the MS. UL bandwidth allocation requests are issued on an MS's Connection IDentifier (CID) basis and UL bandwidths are allocated on an MS basis. That is, the MS requests a required bandwidth for each CID and the BS collects information about requested bandwidths for respective CIDs and allocates the bandwidths to respective MSs. The MS can request a UL bandwidth by polling or piggyback. Polling is a scheme in which the MS requests bandwidth allocation by transmitting a 6-byte Bandwidth Request (BR) header having information about a UL bandwidth required for each CID of the MS at every predetermined polling interval.
FIG. 1 illustrates the structure of a UL BR header. Referring to FIG. 1, the UL BR header includes a Header Type (HT) 101, an EnCoding (EC) 103, a Type 105, a BR 107 and 109, a CID 111 and 113, and a Header Check Sequence (HCS) 115.
Before describing the piggyback scheme, a description will be made of the structure of a generic Media Access Control (MAC) header with reference to FIG. 2.
FIG. 2 illustrates the generic MAC header structure. Referring to FIG. 2, the generic MAC header includes an HT 201, an EC 203, a Type 205 and 206, an Extended Subheader Field (ESF) 207 indicating whether a subheader is included, a Cyclic Redundancy Check (CRC) Indicator (CI) 209 indicating the existence or absence of CRC errors, an Encryption Key Sequence (EKS) 211 including information about a key used for payload encryption, a LENgth (LEN) 215 and 217, a CID 219 and 221, and an HCS 223.
In order to request bandwidth allocation according to the piggyback scheme, the MS sets a Least Significant Bit (LSB) 206 of the Type 205 and 206 to ‘1’ and piggybacks the generic MAC header with a grant management subheader having information about a UL bandwidth required for a certain CID. The Type LSB 206 indicates whether the grant management subheader is set, and an excess bandwidth to be added to an already-allocated bandwidth for the CID is written in the grant management subheader. The contents of the grant management subheader vary depending on a service type corresponding to the CID. The grant management subheader has the following format illustrated in Table 1.
TABLE 1SizeSyntax(bits)NotesGrant management subheader( ) { if (scheduled servicetype==UGS) { SI1 PM1 FU1 FL4 reserved9Shall be set tozero }elseif (scheduled servicetype==extended rtPS) { Extended Piggyback Request11 FU1 FL4 }else { PiggyBack Request16 }
Table 1 illustrates a grant management subheader format for Unsolicited Guaranteed Service (UGS) and extended real time Polling Service (rtPS), by way of example. Depending on a scheduled service type, the grant management subheader varies in contents. Besides UGS and extended rtPS, the grant management subheader can serve such scheduled service types as rtPS, non real time Polling Service (nrtPS), and Best Effort (BE) service.
Fields of the grant management subheader are given in Table 2 below.
TABLE 2LengthName(bits)DescriptionSI1Slip Indicator0 = No Action1 = Used by the MS to indicate aslip of UL grants relative to theUL queue depthPM1Poll-Me0 = No Action1 = Used by the MS to request abandwidth pollFLI1Frame Latency Indication0 = Frame latency field disabledfor this grant1 = Frame latency field enabledfor this grantFL4Frame LatencyThe number of frames previous tothe current one in which thetransmitted data was available.When the latency is greater than15 then the FL field shall be setto 15.Extended11Extended PiggyBack RequestPBRThe number of bytes of ULbandwidth requested by the MS.The bandwidth request is for theCID. The request shall notinclude any PHY overhead. Therequest shall be incremental. Incase of the Extended rtPS, if theMSB is 1, the BS changes itspolling size into the sizespecified in the LSBs of thisfield.PiggyBack16PiggyBack RequestRequestThe number of bytes of ULbandwidth requested by the MS
As described above, to use resources efficiently for data transmission and reception, it is critical that the BS rapidly allocates a UL bandwidth to the MS in response to a UL bandwidth allocation request from the MS.
Due to its fixed polling interval, however, the polling scheme causes a latency when a bandwidth allocation request is generated during a time period between polling intervals. If the MS does not transmit a bandwidth allocation request during a predetermined polling interval, UL resources allocated for transmission of a BR header are wasted.
The piggyback scheme boasts of high utilization and less overhead than the polling scheme since it is feasible whenever UL transmission data exists. Nonetheless, the piggyback scheme also has a shortcoming in that a bandwidth allocation request is confined to a CID included in a generic MAC header (hereinafter, referred to as a generic MAC header CID). Therefore, if the MS corresponding to the generic MAC header CID wants to request bandwidth allocation for another CID that the MS has, it should use a 6-byte BR header, thus increasing overhead.